The invention relates to treads for tires and in particular to the tread patterns of such treads; it also relates to a molding element intended to be fitted on a mould for molding such treads.
In known manner, a tire intended to be fitted on a heavy vehicle comprises a carcass reinforcement surmounted radially to the outside by a crown reinforcement which in turn is surmounted by a tread, the surface radially to the outside of which constitutes the running surface of the tire and is intended to come into contact with the roadway during travel of this tire. To meet the demands in terms of grip and evacuation of water during travel on a wet road, it is known to provide the tread with a plurality of grooves of essentially longitudinal (or circumferential) and transverse (that is to say forming an average angle other than zero with the longitudinal direction) orientation; these grooves define tread pattern elements (ribs, blocks) having an upper face radially to the outside forming part of the running surface of the tread and lateral faces limiting said grooves.
For certain types of tires for heavy vehicles and in particular those intended for use on snow-covered ground, it is necessary to increase the performance in terms of grip and drivability of said tires, whatever their level of wear in order to retain for as long as possible the initial performances of said tires. To increase the grip of a tire, it is known to make at least one incision (that is to say a cutout of width less than or equal to 2 mm and not really creating a hollow volume in the tread) on a plurality of tread pattern elements; the greater the number of incisions and the greater the number of ridges formed on the tread, the more the grip performance is improved. In order to retain for as long as possible the grip performance of the tread as it becomes worn, it is indispensable to provide incisions which are of depths equal to or very close to the depth of the longitudinal grooves (the latter depth being able to define the useful thickness of the tread from the point of view of the quantity of rubber to become worn). This embodiment is unfortunately incompatible with obtaining tread pattern elements having satisfactory rigidity (mainly under the stresses in the contact with the roadway) and may even, under some conditions of travel, result in rubber being torn off the tread (fragility of the rubber slices of low thickness).
An intermediate solution was proposed and marketed by the Applicant (under the name XDN), consisting of producing, in the tread pattern blocks, incisions extending from the running surface when new to a depth substantially equal to half the thickness of said tread. Of course, although this tread pattern permits satisfactory performance when new and until the tread is half-worn, it does not make it possible to maintain this performance for the entire potential period of use of the tire (that is to say until complete wear of the tread has occurred) since the ridges formed by the incisions disappear once wear substantially equal to half the thickness of the tread has occurred.
In the patent U.S. Pat. No. 6,408,910 it is disclosed a tread for a tire having a sculpture which comprises incisions opening on the rolling surface when new and prolonged towards the inside a the tread by a channel intended to become a new groove after partial wear.